Audio power line filter – for what?


Are external electromagnetic interference filters still useful today? Why are not high quality EMI (electromagnetic interference) filters parts of most electronic devices?

Why should we be interested in radio frequency interference even if we do not hear it and see it?

Basic principle

Electromagnetic interference filters suppress (restrict) radio frequency interference going from the power grid to sensitive appliances, usually to audio devices. Electromagnetic interference filters also work the other way, they can suppress HF (high frequency) interference going from the appliance to the power grid. Some modern external electromagnetic interference filters have integrated overvoltage protection and protect very expensive devices from the transient processes that often occur in the power grid. Surge protection limits short-term peaks of voltage in the power grid that may occur as a result of lightning strike, when high wattage is switched on, or the local power grid is switched on or off.

Sources of interference

Nowadays, sources of electromagnetic interference are primarily boost converters everywhere around us. We can find them for example in mobile phone chargers, computers and televisions. They are everywhere you look. In the European Union, all these appliances must comply with statutory EMC standards, see link. For most manufacturers, the basic requirement for constructional solution of their source part is the lowest price. So appliances often balance on the edge of valid EMC standards and “some” HF interference usually escapes to the power grid. Another problem of boost converters is the distortion of the current in the power grid, of the so-called higher harmonic components of the current.

You share the power grid with your neighbours. The power grid is a circuit of low voltage transformer station from which low-voltage electricity is distributed to your home or apartment by wires. You share the same wires with your neighbours. What your neighbour sends into the power grid is transferred to your power sockets by the same wires. Hundreds of households in the housing area together create HF interference, which will reach your appliances through wires you share.

Why filter high frequency interference even if you are not able to hear it?

Modulation and interference

Interference is a characteristic feature of electromagnetic waves. In the electric current the waves are mutually influencing and overlapping each other. At one point the electromagnetic waves intensify each other while at other points they reduce each other. Modulation arises when several frequencies cumulate, in our case the low frequency (audible) is modulated on high frequency. Low frequencies arise by random composition (modulation and interference) of high frequencies.

Types of external power line filters

Let’s not get into design details; usually a LC attenuator is used to lower certain frequency spectrum. Better filters also have integrated overvoltage protection. The most effective filtration is a combination LC filter, overvoltage protection and separator transformer, which will almost completely isolate your appliances from HF interference of the power grid.

How does an isolation transformer work?

Transformer works in such a way that it converts electricity passing through a primary winding into magnetic flow. The electromagnetic flow is transferred from transformer plates to secondary winding by using magnetic circuit, on which voltage is again invoked. Standard transformer plates will greatly reduce frequencies higher than 50Hz, so the HF components are not transmitted on the secondary winding. HF interference is transmitted only by the capacity coupling between the windings, but this can be eliminated by appropriate design solution.


Isolation transformer separates the connected device from the network by galvanisation. However, the protective earth conductor (PE) is still connected to the grid. The socket terminals L and N are now at a separate potential. It means that phase L will not hurt you, of course, provided that you do not touch the N terminal at the same time. The N-terminal must not connect to the protective earth conductor. For more information, please refer to EN 61558-2-4.